“The chemical striping suggests the plume is pulsing, like a heartbeat.” That is how Professor Tom Gernon of the University of Southampton reports a breakthrough which is revolutionizing our knowledge of the creation of continents and the rupture of continents. Beneath Ethiopia’s Afar region, researchers have found periodic pulses of molten mantle rock that someday could split Africa in two and form a new sea.
At the triple junction in Afar, where the Somalian, Arabian, and Nubian plates are pulling apart, the crust is stretched and thinned. Geologists once thought that the mechanism was being driven by a hot mantle plume pulling it apart, but with imperfect details. A research team has now used precise geochemical fingerprinting and high-resolution seismic tomography to the problem and shed light on underlying processes.
Over 130 Afar and Main Ethiopian Rift volcanic rock samples were gathered by scientists. Whole-rock geochemistry and isotopic signatures 87Sr/86Sr, 143Nd/144Nd, and 206Pb/204Pb ratios were employed to quantify repeated chemical bands along the rift system. The bands, or “geological barcodes,” show that the Afar mantle plume is not an even upwelling but pulses up in an asymmetric rhythm. The chemical signals of each pulse are unique, i.e., the mantle source varies with space and time.
The study has far-reaching implications. “We found that the mantle beneath Afar is not uniform or stationary – it pulses, and these pulses carry distinct chemical signatures. These ascending pulses of partially molten mantle are channelled by the rifting plates above. That’s important for how we think about the interaction between Earth’s interior and its surface.” That relationship is not fixed. Rather, the tectonic plates above dictate the operation of the deep mantle, directing volcanic activity to where the crust is thinnest and most stretched.
Seismic tomography has enabled one to see those upwellings. By making use of dense seismic arrays of seismometers and seismic-wave travel-time recordings, researchers have imaged low-velocity zones in the top 200 kilometers of mantle under Afar regions which are interpreted as partial melt supporting surface volcanism. The most melt-sensitive lowest S-wave velocities are found along the segmented rift axis, suggesting the implication not only of the existence of mantle upwelling but of organized, regularly spaced features.
Sensitive statistical modeling using principal component analysis and K-means clustering reaffirmed that a chemically homogeneous, monolithic plume is the best fit to observed trends in geochemistry for each of the three arms of the rift. The pulses themselves adapt as per their nature to the overall tectonic setting. In the Red Sea Rift, where it is extending most energetically and crust is thinnest, the pulses propagate more effectively “like a pulse through a narrow artery,” Gernon describes.
The chemical richness of every pulse isn’t an amenity; it has tangible effect on the surface. When individual pulses arrive at the crust, they create volcanic eruptions and regulate the style and frequency of earthquakes. The connection between plate tectonics and mantle dynamics is so close that, as Dr. Derek Keir suggests, “the evolution of deep mantle upwellings is intimately tied to the motion of the plates above. This has profound implications for how we interpret surface volcanism, earthquake activity, and the process of continental breakup.”
Geochemical fingerprinting is necessary in order to understand the origin and evolution of these mantle plumes. By examining transition elements like Ni and Cr, combined with MgO content, researchers can differentiate between magmatism associated with plumes and upper mantle melts. Elevated Ni (>200 ppm) and Cr (>500 ppm) contents, combined with elevated Fe/Mn ratios, are characteristic of deep mantle plume sources. These signatures, locked into Afar’s volcanic rocks, hold open a window into the chemical evolution of the mantle and its interaction with surface processes.
Such profound structures are not easy to image. Seismic tomography, though capable, needs high station density and quality data to image the slender plume tails. New methods such as floating seismometers over oceanic areas have enhanced resolution, but travel-time measurement uncertainty and vertical smearing remain problems, as observed in synthetic tomography trials.
Afar is a natural laboratory to observe the onset of continental rifting. The rhythmic thumps vibrating under it are not as much geological idiosyncrasies but rather the forces that shape planetary evolution. Over millions of years, they will keep on declining and tear the African continent apart, leading to the eventual flooding with seawater and the formation of a new ocean basin. Each pulse would leave behind a chemical and seismic signature that will be embedded in the rocks, following the rhythm of Earth’s interior for future geologists to decipher.
As Dr. Watts would indicate, “putting together the full picture” of such processes is best accomplished by multidisciplinary exchange, integrating geochemistry, geophysics, and mathematical modeling. The beat of the Afar plume can now be heard, and its rhythm is redrafting the history of how continents split apart, oceans are created, and the deep Earth constructs the world above.
